index.js

function PlaceholderHider(itemClass, placeholderClass){
    var errors = []
    if(arguments.length !== 2)
        throw new Error("Expected two arguments, received " + arguments.length);

    for(var i in Array.prototype.slice.call(arguments)) {
        if(document.querySelectorAll(arguments[i]).length === 0)
            errors.push(arguments[i]);
    }

    if(errors.length > 0)
        throw new Error("Unable to find elements for " + errors.join(', '));

    this.itemClass = itemClass;
    this.placeholderClass = placeholderClass;
    // assuming all placeholders use same initial display
    this.defaultDisplay = document.querySelectorAll(this.placeholderClass)[0].style.display;
}

// use to modify the gcm i.e. to change the numeric value that causes placeholders to disappear
PlaceholderHider.prototype.gcm = (function(value){
    var initialValue = 4;

    return function setter(val){
        if(typeof arguments[0] === 'number') {
            setter.gcm = Math.floor(arguments[0]);
            this.hidePlaceholders();
        }
        if(!setter.gcm){
            return initialValue;
        }
        else
            return setter.gcm;
    };
})();

// unhide placeholders then hide placeholders according to math on gcm
PlaceholderHider.prototype.hidePlaceholders = function hp() {
    var i;
    var items = document.querySelectorAll(this.itemClass);
    var placeholders = document.querySelectorAll(this.placeholderClass);

    var j;
    for(j = 0; j < placeholders.length; j++)
        placeholders[j].style.display = this.defaultDisplay;

    var totalElements = items.length + placeholders.length;
    var k = 0;

    while(totalElements % this.gcm() !== 0) {
        placeholders[k].style.display = 'none';
        totalElements--;
        k++;
    }
};

module.exports = PlaceholderHider;